install_lib should take precedence when reading distutils config.

pip/locations.py

@@ -179,6 +179,10 @@ def distutils_scheme(dist_name, user=False, home=None, root=None):
     for key in SCHEME_KEYS:
         scheme[key] = getattr(i, 'install_' + key)
 
+    if i.install_lib is not None:
+        # install_lib takes precedence over purelib and platlib
+        scheme.update(dict(purelib=i.install_lib, platlib=i.install_lib))
+
     if running_under_virtualenv():
         scheme['headers'] = os.path.join(
             sys.prefix,

tests/unit/test_locations.py

@@ -180,3 +180,17 @@ class TestDisutilsScheme:
         )
         scheme = distutils_scheme('example')
         assert scheme['scripts'] == '/somewhere/else'
+
+    def test_install_lib_takes_precedence(self, tmpdir, monkeypatch):
+        f = tmpdir.mkdir("config").join("setup.cfg")
+        f.write("[install]\ninstall-lib=/somewhere/else/")
+        from distutils.dist import Distribution
+        # patch the function that returns what config files are present
+        monkeypatch.setattr(
+            Distribution,
+            'find_config_files',
+            lambda self: [f],
+        )
+        scheme = distutils_scheme('example')
+        assert scheme['platlib'] == '/somewhere/else/'
+        assert scheme['purelib'] == '/somewhere/else/'